async/await — Asynchronous Programming in Rust

Future trait-based async model with runtime freedom

Syntax is similar to JavaScript's async/await but internals differ. Rust's async fn compiles to a state machine implementing the Future trait. Heap allocations minimized.

async fn fetch_data(url: &str) -> Result<String, Error> {
    let response = reqwest::get(url).await?;
    let body = response.text().await?;
    Ok(body)
}

Rust's standard library includes no async runtime. Users bring tokio (most popular) or async-std. This design lets embedded environments use lightweight runtimes while servers use high-performance ones.

Pin and Self-Referential Futures

When async blocks hold references to local variables across .await points, self-referential structures form. Pin exists to handle this safely. This is the hardest part of Rust async.

Key Points

async fn compiles to a state machine implementing Future trait

.await yields control to runtime — other tasks run

#[tokio::main] initializes runtime + makes main async

Pros

✓ Zero-cost — Futures compile to state machines, minimal heap allocation
✓ Swappable runtimes — tokio, async-std, smol, etc.

Cons

✗ async traits were unstable until recently (stabilizing via RPITIT)
✗ Pin/Unpin concepts are difficult
✗ Ecosystem split between tokio vs async-std libraries

Use Cases

Web servers (axum, actix-web) — handle thousands of connections with few threads HTTP clients (reqwest) — concurrent multiple requests